The present invention relates generally to radiography, and more particularly to using radiographic inspection for examining objects.
Radiographic inspection generally involves discriminating an amount of penetrating radiation (i.e., x-ray or gamma ray) that is transmitted along different paths through an object. Current radiographic inspection methods are capable of only detecting the total amount of radiation being transmitted along a path through the object and not the amount of radiation being transmitted at each of the individual locations that comprise the path. For example, in a conventional x-ray radiographic system, a sheet of film is used to detect the x-rays being transmitted through an object along the paths that arise between each region of the sheet of film and a x-ray source. In regions corresponding to paths that contain relatively large amounts of highly attenuating material, the film intercepts few transmitted x-rays, whereas for paths containing small amounts of less attenuating materials, the film is heavily exposed. Since the sheet of film is sensitive to the total of summed contributions to the beam attenuation along each path, it is very difficult to discriminate between paths containing a small amount of highly attenuating material and paths with a correspondingly greater amount of less attenuating material.
The lack of discrimination in conventional radiographic methods makes detection of certain types of flaws extremely difficult. In particular, it is very difficult to detect defects in objects that are characterized by having small amounts of slightly attenuating materials combined with an overall background of highly attenuating material. Examples of such inspection problems are present in x-ray radiographic detection of nitrogen contamination of titanium, which is believed to be the underlying cause for hard .alpha. defects, and the detection of unremoved ceramic core material in cast nickel-alloy turbine blades, which is believed to be responsible for certain types of blade failure during operation. However, the contaminants in both these examples attenuate neutrons at significantly different levels then the base materials. If the titanium nuggets and turbine blades contain small amounts of neutron attenuating material, it is very important to be able detect nuggets and blades which have the neutron attenuating material. In either case, conventional x-ray radiographic detection is unable to discriminate the neutron attenuating material from the large x-ray attenuation material.
One possible solution to the above problems is to use computed tomography (CT), which measures the local attenuation coefficient in an image plane. However, CT requires a large number of radiographic exposures for each image plane and extensive computation for reconstructing the image. Other disadvantages in using CT for inspections are that it is expensive and time-consuming, especially if many image slices are required to adequately sample the object volume.
Another technique used to overcome the problems associated with conventional radiographic methods is to use dual-energy x-ray methods to discriminate between material properties. Dual energy x-ray methods attempt to measure the amount of energy that is absorbed and scattered. Determining the separate contributions between absorption and scattering requires a significant amount of processing, and is generally very difficult to do effectively with conventional x-ray sources. In order to overcome this problem, some dual energy methods have replaced the variable energy x-ray source with gamma ray sources. However, gamma ray sources are usually weaker than x-ray sources, resulting in greater signal to noise effects. A problem common to both the x-ray source and the gamma ray source dual-energy method is that they are not sensitive to neutrons.
Another technique that has been used is to replace the radiographic film with gas ionization detectors filled with BF.sub.3, .sup.3 He or Xe. An example of a gas ionization detector filled with Xe is disclosed in U.S. Pat. No. 4,570,071. The problem with these types of gas ionization detectors are that they are sensitive either to only x-rays or gamma rays or only to neutrons and not sensitive to both x-rays or gamma rays and neutrons.